Deposition of organic waste materials often
leads to significant problems due to their instability. It is known
that anaerobic degradation develops about 100 kg/ton of methane, but
according to the Kyoto convention it is allowed to deposit organic
materials only in sanitary dumps. At the same time, during the
decomposition of these materials the volume of discharged waste waters
is significantly increased as well as unpleasant odor emission around
the dump site.

The need to process the waste of organic
origin (food waste, bio waste) in the aim to reduce the existing dumps
and to revitalize part of their content, has led to significant
development of the composting technology, i.e., mechanical-biological
processing. Table 1. displays the overview of the most common
components of municipal solid waste, and their physical
characteristics. We find 96 chemical elements in nature, but only 18
elements participate in plant structure, of which the most important
are hydrogen, oxygen, nitrogen, carbon, phosphorus and sulphur. All
elements except carbon, plants obtain from the soil, so that in time
depletion of these elements occurs, i.e., they need to be restored.
Compost incorporates many of these elements, thus implying its
usefulness. Naturally, which compost and in what concentration should
be used for particular soil and particular plant culture depends on the
features of land such as porosity, humidity, pH value and other
chemical and biological parameters.
Compost use decreases land erosion and pollution caused by
overtreatment with artificial fertilizers.

Municipal waste component

Concentration

kJ/kg

Carbon

Evaporable
content

Nonevaporable
content

Humidity

Paper and cardboard

41%

13490

35%

37%

44%

19%

Bio waste

15.3%

9300

23%

21%

33%

46%

Food waste

6.8%

7560

18%

17%

23%

60%

Plastics

9.8%

26980

56%

20%

65%

15%

Wood

3.8%

16050

41%

60%

44%

16%

Textiles

2.2%

15350

37%

36%

39%

25%

Rubber, leather

2.4%

19538

43%

24%

66%

10%

Glass

7.6%

0

0%

0%

100%

0%

Metal

9.0%

0

0%

0%

100%

0%

Mix

2.1%

0

25%

33%

33%

33%

Average
value

100%

11235

27.5%

24.7%

50.9%

24.4%

Table 1.

2. Definition of Composting

Composting is a controlled biological
decomposition and stabilization of organic substrate, under the
condition that it is primarily aerobic and that it allows for the
development of thermophilic temperature as a result of biologically
generated heat. The process generates the final product which is
sanitary and stabilized, has high concentration of humus substances and
can be usefully applied to the land.

In other words, composting is a natural
process that transforms organic waste into clean and useful product.

3. Composting Process

Composting process is the application of a
natural process of putrefaction. Significant difference is in that
enough volume of organic material should be gathered and that in this
way generated heat is preserved. This has a favorable effect on the
process speed and performs selective sterilization of potential
pathogens and weed seeds known as sanitation, pasteurization or
disinfection.

Composting is performed under the influence
of microbiological processes. Commonly the process is divided into four
phases: heating, thermophile decomposition, mesophile decomposition and
maturation.This classification
corresponds to
temperature ranges under which certain microorganisms develop or are
especially active.

Figure 1. Temperature changes (full line),
population of mesophile fungi (dashed line) and thermophile fungi
(dotted line) in compost. Left ordinate indicates the logarithm of the
volume of formed fungi colony per gram of compost and the right
ordinate - temperature at compost center

4. Optimization of the Composting Process

Efficient composting needs appropriate raw
material base which is "food" for microbes that initiate the process.
Sufficient amount of easily degradable carbon is necessary. This is
sometimes called the "evaporable solid content" (Table 1.) and is
important for fast heating.

Materials with high nitrogen
content

C:N

Cut grass

19:1

Mud digested

16:1

Food

15:1

Cow manure

20:1

Horse manure

25:1

Materials with high carbon content

C:N

Leaves

40-80:1

Tree bark

100-130:1

Paper

170:1

Wood and saw dust

300-700:1

Table 2.

Also it is necessary to achieve balance of
other materials, for example nitrogen. This precondition is often
designated as the C:N ratio. Table 2. lists some of the materials that
are processed during composting.

Composting is an aerobic process, and
sufficient volume of oxygen must be applied in order to enable process
realization. Air must be enabled to reach all parts of the deposits. At
the overall level this implies application of a rotating mechanism or
injecting air. Water concentration is also a critical factor. Water
concentration should be between 40-60%. Higher humidity levels
decreases porosity which disables the maintenance of the necessary
oxygen level. At a lower humidity level biological activity is
insufficient for process continuation.

For efficient microbiological activity,
these conditions should be present at the micro level rather than on
the overall score. Necessary parameters of waste material and process
are:

Standards differ by their content, but
generally they require that products are safe for humans to handle and
for the environment which is being fertilized by it. Standards can also
have components that imply quality appropriate for application.

Usually components of standards include:

contamination with heavy metals and chemicals. Heavy metals
are omnipresent in the environment, but can be a problem in compost,
especially those separated from biosolids and mud for processing.
Chemical contaminants such as pesticides are also limited by some
standards.

Hyman pathogens: Typically salmonella and coliform are also
limited. Since these requirements can be determined by direct testing,
commonly specific temperature of the composting process is demanded.
Typically it is 3 days above 55°C.

Weed seeds : composting process should destroy majority of
the seed population, this aspect is also controlled by some standards.

Reduction of the attraction vector: degradable waste
is usually attractive for a whole range of disagreeable animals
including birds and rodents. Complete composting eliminates attraction
of the material. This requirement is usually based on time and
temperature.

decreased plant growth: As discussed above, compost that
has not matured can be toxic for plants. Herbicide transfer is also a
problem occasionally. Obviously this is a significant factor where the
product is used as a component of an agent for plant growth.

Physical contaminants: metal, glass, plastics and gravel
are possible contaminants of compost and their concentration reflects
the quality of the raw material base.

Facility is based on the Bed minster's
procedure and is fully automated.

Municipal solid waste is sorted following
waste receipt whereby
materials that can be recycled such as paper, plastics, ferromagnetic
materials, aluminum particles, and non-biologically degradable
materials are extracted. Material obtained in this way is directed to
the reactor where it undergoes mechanical-biological treatment whose
final objective is compost generation.

From the start of processing compost is
isolated from the environment
so that permanent monitoring and high quality control is possible. At
the same time, since the process is fully automated, it is possible to
perform continuous treatment and maximum manufacturing productivity. It
is necessary to stress that the compost generating process is performed
continuously in the reactor that rotates at 1 rotation per minute.
After 1800 rotations the compost is screened, pulverized, freed from
metal particles, gravel and other heavy components and is directed for
fermentation and maturation.

Facility performance

Rated yearly capacity for
processing of municipal waste

25000 t

Maximum daily capacity (
mud and municipal waste)

100 t

Necessary yearly mud volume

2000 t

Yearly compost output

10000 t

Bio reactor parameters

Length

42 m

Diameter

4.2 m

Wall thickness

16 mm

Bio reactor mass

222000 kg

Number of rotations of the
reactor pipe

1 min-1

Plant rotation power

2x75 kW/ 1500 min-1

Technological units of the composting
facility are displayed on the following diagram: